1. Field of the Invention
The present invention relates to a dust filter for trapping dust in air, and in particular, to a device for detecting a degree of clogging of the dust filter and producing a signal indicating same.
2. Description of the Related Art
Known in the prior art is a dust filter constructed by a rough mesh portion and fine mesh portion, a thermo-sensitive electric resistor arranged in each of these mesh portions, and a bridge circuit constructed by these thermo-sensitive resistors. The bridge circuit is connected, at terminals thereof to which a voltage is not supplied, to a warning device that provides a warning when a ratio of the values of the thermo-sensitive resistors is greater than a predetermined value. (See, Japanese Utility Model Publication No. 55-65119.) Namely, in this prior art, a special portion of a rough mesh is provided in a usual fine mesh portion of the dust filter, a value of an electric resistance at this rough mesh portion is obtained by measuring a velocity of a flow of air passing through the rough mesh portion, and a value of an electric resistance at the fine mesh portion is obtained by measuring a velocity of a flow of air passing through the fine mesh portion, whereby a ratio of the value of the measured electric resistance at the rough mesh portion to a value of the electric resistance at the fine mesh portion is calculated from the velocity of a flow of air passing through the rough and fine mesh portions.
This prior art suffers from a drawback that, to detect a degree of clogging of the filter, a dust filter having a special design, i.e., in addition to a usual fine mesh portion, a rough mesh portion must be provided in the dust filter, must be used. Accordingly, there is a demand for an improved device capable of detecting a degree of clogging of a dust filter having a normal design.